1. Field of the Invention
The present invention relates to a power supply device that includes a number of batteries serially connected to each other for supplying high output voltage, and in particular to a power supply device for suitably driving vehicles.
2. Description of the Related Art
Vehicle power supply devices include a plurality of batteries that are serially connected to each other to increase the output voltage of the power supply devices. In such power supply devices, the serially-connected batteries are charged at the same amount of charging current, and are discharged at the same amount of current. For this reason, if all the batteries have the completely same properties, their battery voltages and remaining capacities will not be unbalanced. However, practically, batteries with completely the same properties cannot be manufactured. The unbalance among the batteries causes uneven voltages and remaining capacities after the batteries are repeatedly charged/discharged. In addition, if the batteries have unbalanced battery voltages, a particular battery may be over-charged or over-discharged. The over-charging/over-discharging operation may cause not only battery deterioration but also reduction in battery safety. A power supply device has been developed that, in order to prevent this problem, detects the voltages of batteries and eliminates the voltage difference among the batteries.
See Japanese Patent Publication No. 4029351, Japanese Patent Laid-Open Publication No. 2003-333762, and Japanese Patent Publication No. 4244531.
In the power supply device disclosed in Japanese Patent No. 4029351, a discharging circuit is connected in parallel to batteries that compose series battery groups. The discharging circuit discharges a battery the voltage of which is high to reduce the voltage of this batter. Thus, the unbalance among the batteries is eliminated so that the properties of the battery are equalized.
In the device that discharges the batteries so that the properties of the battery are equalized, a battery with high remaining capacity or voltage is discharged with reference to a battery with the lowest remaining capacity or voltage. Since the voltage of a battery with small remaining capacity is low, for example, the equalization circuit discharges batteries with higher voltage in order to equalize the voltages of the batteries so that the batteries with higher voltage are brought to the same voltage as the battery with low voltage. Accordingly, in the power supply device that includes a number of batteries serially connected to each other, if the remaining capacity of a particular battery becomes small and the voltage of the particular battery becomes low, the rest of batteries of the voltage of which is higher than this battery are required to be discharged to equalize the voltages of all of the batteries. In this case, since a number of batteries are discharged, the total discharged capacity of the rest of batteries to be discharged will be large. In addition, since a large number of, the rest of batteries to be discharged are reduced to the lowest voltage, the battery voltage unbalance tends to be large. As a result, the capacity to be discharged from batteries with higher voltage will be large. If the total discharged capacity is large, or if the discharged capacity of a particular battery is large, the amount of discharging heat generation will be large. The reason is that the amount of heat generation is proportional to the discharged capacity. In the power supply device disclosed in Japanese Patent No. 4029351, since a battery with high voltage is discharged via a discharging resistor, the amount of heat generation of this discharging resistor will be large. In particular, if the amount of heat generation of a number of discharging resistors is large, it is difficult to effectively dissipate heat generated by the discharging resistors mounted on a circuit board. In addition, since high power consumption discharging resistors have a large exterior shape, actually, it is difficult to mount a number of such discharging resistors on the circuit board. For this reason, actually, it is difficult to mount the large power consumption discharging resistors on the circuit board. Time required to discharge batteries to a lower voltage depends on the power consumption of the discharging resistor. The reason is that the discharging current relates to the power consumption of the discharging resistor. For this reason, it is difficult for a small discharging resistor to be mounted on the circuit board to quickly reduce the voltage of the battery. Accordingly, in this case, there is a disadvantage that it takes time to equalize the voltages of the batteries. That is, there is a trade-off relationship between time required to equalize the voltages of the batteries and the amount of heat generated by the discharging resistor. For this reason, both cannot be satisfied.
The present invention has been developed to solve the disadvantage in the power supply device that includes a number of batteries serially connected to each other. It is a main object of the present invention to provide a power supply device that can reduce total electric power in battery equalization but can quickly equalize the electrical properties of a number of batteries.
It is another main object of the present invention to provide a power supply device that reduces wasted power consumption in equalization among batteries for efficiently equalizing the electrical properties of the batteries.